Influence of bubbles and sand on chlorophyll-a fluorescence measurements in the surfzone

Melissa M. Omand, Falk Feddersen, David B. Clark, Peter J.S. Franks, James J. Leichter, R.T. Guza

Limnol. Oceanogr. Methods 7:354-362 (2009) | DOI: 10.4319/lom.2009.7.354

ABSTRACT: Continuous chlorophyll-a (Chla) measurements in the surfzone (region of wave-breaking adjacent to the shoreline) would increase understanding of harmful algal blooms, food supply for intertidal invertebrates and fishes, and the fate of terrestrial runoff pollution. Optical measurements of Chla fluorescence in the surfzone are affected by bubbles and suspended sand. Here, errors in surfzone Chla fluorescence measurements (using WET Labs ECO Triplet fluorometers) are estimated by comparing observed (Chlaraw) with known (Chlatrue) Chla concentrations in laboratory tests with controlled amounts of bubbles and suspended sand (characterized with concurrently measured optical turbidity, t). For both bubbles and sand, Chlaraw and t are linearly correlated, and the regression line slope depends on Chlatrue. When Chlatrue is low, Chlaraw is biased high, and when Chlatrue is high, Chlaraw is biased low. Fluorometers were also deployed in a natural surfzone, and for the limited range of field Chla observed, the field and laboratory t-Chla relationships were largely consistent. Mechanisms responsible for these biases are proposed, and correction procedures using the observed t-Chla relationship are developed and applied to surfzone Chlaraw observations. For the moderate Chlatrue concentrations (2–4 µg L–1) encountered, errors in hourly mean and instantaneous Chlaraw are less than 5% and 15%, respectively. Larger errors are expected for Chlatrue outside this range. Although further testing is needed, the results suggest that in situ, optical Chlaraw from other turbid environments (e.g., estuaries, bays) should also be interpreted cautiously.